Document inscribed on March 6 1851 by Daniel Kilby, Customs Collector of the District of Passamaquoddy, where he certifies that Isaac N. Sprague, an American Seaman and a native of Pembroke, Maine, about the age of 16, is a Citizen of the United States.
From brief online research, hopefully correct - corrections and/or additions gratefully requested in the comments:
Isaac N. Sprague
Isaac N. Sprague may have been Isaac Noble Sprague (1834-1880), born 22 September 1834 at Pembroke, Maine, son of William and Tamar (Clark) Sprague.
In light of the certificate above, it's tempting to think that Isaac might have crewed on one of the many vessels carrying hopeful miners from the East to the goldfields of California. And, like many other seamen, at some point took stock of the situation and decided to try his own luck at finding gold rather than return East as part of the crew.
Daniel Kilby (1791-1860)
According to a mention in Eastport and Passamaquoddy: A Collection of Historical and Biographical Sketches, compiled by Daniel Kilby's son William Henry Kilby (1820-1898) and published by E. E. Shead in 1888:
After the war, Kilby became a shipowner and merchant. In 1818 he married Joanna Hobart (abt 1800-1848), daughter of Isaac Hobart and Joanna (Hersey) Hobart.
From Eastport and Passamaquoddy: A Collection of Historical and Biographical Sketches: